It is known to imbed sorption or chemisorption masses between webs of fleece as formed shaped bodies in order to obtain filter mats usable as filter media. This is done by linking together two fleece webs containing the bulk material and imbedding the filtering masses between them. In order to maintain the filtering masses in position, the fleece webs are connected to each other so that small, cushion-shaped protrusions having a waffle-like or honeycomb structure result. This way, such a filter mat is of uneven thickness over the surface of the mat. This has the disadvantage that when a gas flows through such a filter mat for purposes of purification, different dwell times of the gas in the filter mat result, depending on the location where the gas passes through the filter mat. Furthermore, an increased gas passage takes place at the thinner locations with lower filling height, as well as at the connection points between the two fleece webs. Therefore, such filter mats are poorly suited to retain noxious substances with a good separation degree.
In order to avoid these disadvantages of the known filter mats, it has already been proposed to insert shaped bodies in the spaces between the cushion-shaped protrusions above the connection strips or to imbed in these areas premolded gratings of plastic material. However, these steps require considerable additional expense whose effects remain still unsatisfactory. Besides, the effective filtering surface is considerably reduced with respect to the available surface. Also the contact volume per space unit is reduced. A further drawback of these filter mats with waffle-like structure consists in the fact that the two fleece webs have to be particularly rigid in order to maintain their geometric shape, especially after they are built in, since a loss of shape can create hollow spaces through which the gas can escape.